A good place to start is Trailfinders. While we book all our US travel ourselves, we use Trailfinders for Asia as they seem to have great fares and hotel deals.
Basically it's cheapest when the weather could be bad (around September) and is particularly pricey around Christmas.
Flights start at around €700. Hotels are really cheap out there too. You can get 5* hotels for the price of a B&B here. There are cheaper hotels too but if I'm going all that way and luxury is affordable then luxury it is!
As for where to visit, Bangkok is a must, it's an experience. I loved it though I know others who didn't quite fall in love with it. After that it's a matter of whether you want to take internal flights and head to the islands, or travel overground to either Pattaya or Hua Hin.